Developing a latent electrostatic image with ferromagnetic carrier and toner by employing a varying magnetic field



Dec. 8, 1970 MASAMICHI smo 3,545,953

DEVELOPING A LATENT ELECTROSTATIC IMAGE WITH FERROMAGNETIC CARRIER AND TONER BY EMPLOYING A VARYING MAGNETIC FIELD Filed Feb. 9, 1968 INVENTOR MASAMICHI SATO ATTORNEYS United States Patent O DEVELOPING A LATENT ELECTROSTATIC IMAGE WITH FERROMAGNETIC CARRIER AND TONER BY EMPLOYING A VARYING MAGNETIC FIELD Masamichi Sato, Saitama, Japan, assignor to Fuji Photo Film Co., Ltd., Kanagawa, Japan Filed Feb. 9, 1968, Ser. No. 704,296

Int. Cl. G03g US. Cl. 961 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND Field of the invention The present invention relates to an improvement of the cascade method and apparatus of developing an electrostatic latent image.

Description of the prior art Xerographic processes are described in US. Pat. No. 2,297,691 and No. 2,357,809, wherein a Xerographic plate composed of a conductive base plate and a photoconductive insulating layer thereon is uniformly charged on its surface. Such uniform charges on the photoconductive insulating layer are discharged by the projection of an image corresponding to an original to be reproduced, thus forming an electrostatic latent image. To develop this electrostatic latent image by carriers accompanying colored powders (hereinafter called toners), the toners are caused to produce friction charges. The toners are brought into contact with the surface of the photoconductive insulating layer and electrostatically retained on the portion of the photoconductive insulating layer corresponding to the electrostatic latent image. Next, the image thus developed is transferred to another appropriate supporting medium such as paper or a transfer material and fixed by an appropriate method. To bring the toners into contact with the surface of photoconductive insulating layer, cascade developing processes are adopted. Under such processes, carrier particles accompanying the toners (hereinafter called cascade developer) are cascaded over a photoconductive insulating layer and caused to electrostatically adhere thereto. Satisfactory results can be obtained by this process if the original comprises letters or line images on a white background. However, in the case of an image comprising a wide black area or continuous gradation, so-called edge effect and other unsatisfactory results are encountered.

As a means to eliminate such defects and to accomplish the faithful development which follows an electrostatic latent image, the use of a developing electrode is known. However, if a developing electrode is placed close enough to a photoconductive insulating layer in an automatic continuous apparatus, the flow of cascade developer is greatly impeded, resulting in the reduction of developing speed and the clogging of developer.

SUMMARY In the present invention, a developing space is maintained between an insulating material such as photoconductive insulating material which bears an electrostatic latent image and a non-ferromagnetic developing electrode. A cascade developer containing ferromagnetic carriers is fed through that space. A magnetic field which varies in terms of time and position is exerted in the aforesaid developing space to carry the cascade developer downward between said interval under the forces of gravity acting upon the cascade developer, electrostatic force of the electrostatic latent image, frictional resistance with the surface bearing the electrostatic latent image and the magnetic force of aforesaid magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary diagrammatic cross-sectional side view of a device arrangement illustrating the principle of the present invention;

FIG. 2 is the vector diagram showing the force acting upon a carrier particle contained in developer in FIG. 1;

FIG. 3 is a fragmentary diagrammatic cross-section side view of another embodiment according to the invention; and

FIG. 4 is an enlarged sectional view showing the details of another developing electrode and different from that shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, electrophotosensitive material 11 comprises a photoconductive insulating layer 12 and a conductive base plate 13. A non-ferromagnetic developing electrode 14 is spaced from and opposite to photoconductive insulating layer 12 of electrophotosensitive material 11. Electromagnets 15 produce an AC magnetic field in the developing space between electrophotosensitive material 11 and electrode 14. The electromagnets are excited by a coil and AC power source (not shown).

Photoconductive insulating layer 12 of electrophotosensitive material 11 is charged uniformly when exposed at areas corresponding to dark areas on the original to be reproduced, thus forming an electrostatic latent image. Electrophotosensitive material 11 is maintained in parallel with developing electrode 14 as illustrated in FIG. 1, and cascade developer 17 is fed from container 16 through the developing space between electrophotosensitive material 11 and developing electrode 14. Cascade developer 17 rolls down by its own weight and is collected in a receptacle 18, leaving toners adhering electrostatically to the electrostatic latent image on the photoconductive insulating layer 12. Cascade developer 17 contains ferromagnetic carriers subjected to the magnetic force of electromagnets 15.

Generally, the electric field of an electrophotosensitive material due to an electrostatic latent image is remarkably strengthened by a developing electrode. Even in cases where no developing electrode is used, the carriers falling under the force of gravity adhere to the surface of the photoconductive insulating layer due to strong electrostatic force. If a developing electrode is used, the clearance between the developing electrode and the photoconductive insulating layer is normally quite small. If the carriers adhere to the latter, the path of the developer becomes clogged and the apparatus fails to operate.

The present invention eliminates this clogging problem. Owing to the effect of the varying magnetic field produced in the space between the photoconductive insulating layer 12 and the developing electrode 13 by electromagnets 15 and to the electrostatic field due to the electrostatic latent image, cascade developer 17 is attracted towards the developing electrode 14 at one moment and toward photoconductive insulating layer 12 at another as it rolls down through the developing space. Even if it were assumed that due to the strong electrostatic force of the electrostatic latent image, carriers of cascade developer 17 have adhered to the surface of photocoductive insulating layer 12, they will be freed the next moment by the strong magnetic force of electromagnet 15. Accordingly, clogging of developer caused by the adhesion of carriers to the surface of photoconductive insulating layer is eliminated.

The following components of force are shown acting upon a carrier particle in FIG. 2: gravity W; electrostatic force F due to electrostatic latent image; magnetic force F due to electromagnet and frictional force N between a carrier particle and the photoconductive insulating layer 12. If electrophotosensitive material 11 is tilted at an angle 0 with respect to the horizontal, the force by which gravity W causes the carrier particle to fall along the photoconductive insulating layer 12 will be W sin 0 and the force perpendicular to the photoconductive insulating layer 12 will be W cos 0. Accordingly, the frictional force tending to check the fall of carrier particles is given by N=,u. (F +W cos 0-F if a equals the coetficient of friction between the carrier and the photoconductive insulating layer 12. As the magnetic force F varies with time, W sin 0 is less than N when F is the strength of electromagnet near zero and the carrier particles will tend to stick to photoconductive insulating layer 12. If the magnetic force F is at its maximum, W sin 0 is greater than N and the carrier particles will fall smoothly. When the magnetic force P is near zero, the adhesion of carrier particles is also reduced in practice due to the inertia of drop. As the carrier particles roll down the slope in this way, the developer runs down while developing the electrostatic latent image. Excellent image can be I obtained by properly selecting the size and weight of carriers, intensity of magnetic field and frequency of AC magnetic field, etc.

FIG. 3 shows a xerographic apparatus according to the present invention in which is a xerographic drum consisting of a conductive drum 21 and a photoconductive insulating layer 22. A non-ferromagnetic developing electrode 23 is placed along the periphery of xerographic drum 20 while maintaining a small clearance with respect to the photoconductive insulating layer 22. Electrode 23 extends over the distance required for development and carries electromagnets 24 to produce an AC magnetic field. A developer feeding port 25 and receptor 26 used for developer are provided. First, the photoconductive insulating layer 22 is charged uniformly at dark places of the original by means of a charging device not shown) to form an electrostatic latent image by a scanning exposure device (not shown). Next, cascade developer is poured from developer feeding port 25 into the clearance between developing electrode 23 and photoconductive insulating layer 22. As the carrier particles in the cascade developer contain ferromagnetic material, they are attracted either in the field of electromagnet 24 towards the developing electrode 2.3 or by the electrostatic latent image. While being alternately attracted towards the electrostatic latent image on the photoconductive insulating layer 22 and the electromagnet 24 behind developing electrode 23, the carrier particles and developer roll down and develop the electrostatic latent image. If the strength and frequency of electromagnet 24 are selected properly, carriers smoothly pass through the clearance between the photoconductive insulating layer 22 and the developing electrode 23 without adhering thereto. The toners sticking to the carriers are nonferromagnetic substance and therefore adhere to an electrostatic latent image under action of electrostatic force independently of the intensity of magnetic field. In cases where a drumshaped electrophotosensitive material is employed as in the case of this embodiment, the farther the developer rolls down, th smaller becomes W cos 6' mentioned in the above description of FIG. 2 whereas W sin 0 becomes the larger. For this reason, the intensity of magnetic field may be made progressively smaller to satisfy the requirement of W sin 0 N for the fall of developer. Developer completing the development in this way falls into the receptor 26 and is collected, circulated and reused by other mechanism (not shown).

FIG. 4 illustrates a modification of the device shown in FIG. 3 in which the end of the developing electrode 23 is extended slightly and a strong electromagnet 27 is arranged in that extended portion. Carriers are completely prevented from sticking to the electrostatic latent image by the strong attraction of electromagnet 27. To prevent the carriers which have been removed in this manner from again being attracted to the electrostatic latent image, it is preferable that the distance between said end portion of the developing electrode and the photoconductive insulating layer in the magnetic field of this electromagnet 27 is larger than in other portions.

The number and shape of the electromagnets 24 are so selected that the density of fiux of magnetic induction varies greatly with the positions in the developing space between photoconductive insulating layer 22 and develo ing electrode 23 while the strength thereof is so selected that the developer fully comes in touch with the surface of drum 21 and the carriers do not stick to photoconductive insulating layer 22 under action of the electrostatic force of the electrostatic latent image. The nearer an AC magnet is to the developing space, the smaller excitation power is sufficient to obtain a magnetic field of required intensity. Therefore, the developing electrode should be made as thin as possible while maintaining mechanical strength.

The carriers of developer to be employed in the practical application of the present invention must contain ferromagnetic substance, at least in their surface or interior. As the toners and carriers of developer are charged respectively according to the relative positions in the friction charging order, those carriers having ferromagnetic metal cores covered with material appropriate in terms of friction charging order are quite satisfactory. For instance, carriers having nickel balls as the ferromagnetic metal cores and coating with ethylcellulose have specially excellent characteristics. As for the ferromagnetic metal core, balls made of iron, chrome, cobalt,

etc., can also be employed. Needless to say, ferromagnetic metal balls made of iron, nickel, chrome. cobalt, etc., may be used by themselves as the carriers. Also, the carriers containing ferromagnetic powders are applicable; for instance, iron, nickel, chrome, cobalt or other metal powders such as manganese-bismuth powders, etc., as burnt or dispersed in resin and formed in globular shape can be employed. Further, their surfaces may be coated with a material being appropriate in terms of friction charging order. Moreover, non-ferromagnetic cores covered with ferromagnetic material, or cores thus covered and further coated with a material excellent in terms of friction charging order may be used. For example, miniature balls of lead or copper, etc., with large specific gravity being employed as the cores and plated with iron, nickel, chrome or cobalt. etc., or coated with ferromagnetic powders dispersed in resin, or such coated cores whose surface is further covered with appropriate resin are also effective.

If only the electrostatic field due to an electrostatic latent image is taken into consideration, smaller clearance between a developing electrode and the surface of a photoconductive insulating layer is more desirable, whereas larger clearance is more desirable if importance is attached only to the smooth fiow of developer. In practice, as small a clearance as possible is preferable as long as clogging of developer is not caused thereby. Satisfactory results can be obtained by selecting the clearance within a range from a value slightly larger than the maximum diameter of developer carrier to the one about several times as large as the said diameter.

As regards the electric current to be supplied to the coil of electromagnet to produce AC magnetic field, not only sine wave, but also square wave and pulse wave, etc., or current varying with time such as pulsating current may be applied. To minimize the heating of coil, it is preferable to apply pulse current having a short duration time.

Under the developing method of apparatus of the present invention, it is possible to achieve a satisfactory dry type reproduction of an image comprising a Wide black area or continuous gradation by placing a developing electrode close to an electrophotosensitive layer While maintaining the simplicity as well as the reliability of the conventional cascade developing process. Further, as the developer does not hit strongly against a photoconductive insulating layer, it is possible to remarkably extend the life of the photoconductive insulating layer. Also, the carriers of developer need not always be globular in shape and though globular ones generally pass through a narrow gap more smoothly, the passage of developer takes place smoothly under the present invention owing to the efiect of magnetic field changing with time, independently of the configuration of carriers.

What is claimed is:

1. A method of developing an electrostatic latent image on an insulating material under an electrophotographic cascade developing process comprising the steps of: feeding cascade developer containing ferromagnetic carriers through a developing space between a photoconductive insulating material bearing an electrostatic latent image and a non-ferromagnetic developing electrode spaced from said material, said developing space including an alternating current magnetic field; and varying the magnetic field with time and position in said developing space in such a manner to alternately cause said carriers to be attracted towards said insulating material and said developing electrode.

2. The method of claim 1 where the strength of said magnetic field varies inversely with the force due to gravity tending to hold said cascade developer to said develop ing electrode.

3. The method of claim 1 where said magnetic field is stronger at the trailing edge of said developing electrode than in any other place thereof.

References Cited UNITED STATES PATENTS 1/1964 Lehmann 118637 8/1968 Donalies 118-637 US. Cl. X.R. 

